Secondary battery power storage system

Abstract: The present invention provides a distributed secondary battery type power storage system capable of maintaining the soundness of the secondary battery and of efficient electric power charging and discharging operations. The secondary battery power storage system comprises a secondary battery connected to a load, a charge/discharge unit connectable to a power system and connected to the secondary battery, and a plurality of loads connected to the charge/discharge unit. A power receiving object is selected and a capacity is determined on the basis of information about the operating condition of the plurality of loads, and the surplus electric power remaining in the secondary battery after feeding electric power from the secondary battery to the load is fed to the selected power receiving object through the charge/discharge unit. The secondary battery power storage system is capable of maintaining the soundness of the secondary battery and of efficient charging and discharging operation.

Description: Background of Invention: Field of Invention: The present invention relates to a secondary battery electric power storage system and, more particularly, to a battery system which is prouided with evaluation meams of the soundness and the residual power of a battery, the distribution of the surplus electric power of a battery, and the optimum charge/discharge control of a battery. Most secondary batteries for automobiles, uninterrupted power supplies and the like have been lead-acid batteries. The diffusion of electric power storage systems and electric vehicles, and the development of secondary batteries capable of storing electric power in a high energy density have been strongly desired to meet rising necessity for the leveling of power demand and progressively increasing demand for global environmental protection, and the development of new batteries has been expected. Various kinds of large-capacity secondary batteries including nickel cadmium batteries, nickel-metal hydride batteries, lithium batteries and sodium-sulfur batteries will come onto the market in the future for various purposes. When managing batteries, appropriate charging and discharging of batteries according to their characteristics are essential to securing the soundness and the long life of batteries. Each battery has specific charge/discharge characteristics, and temperature characteristic, rate characteristic and self-discharge characteristic vary from battery to battery. In view of the convenience of using the battery, the residual capacity of the battery must be known as accurately as possible. Charging systems, temperature management and methods of determining the residual capacity for each of those batteries have been proposed. There have been known, for example, methods of detecting the residual capacity on the basis of only voltage (Japanese Patent Laid open (Kokai) Nos. 58-85179 and 61-135335), a method of managing the residual capacity on the basis of voltage and current (Japanese Patent Laid-open (Kokai) No. 52-32542), methods of managing the residual capacity on the basis of current and time (Japanese Patent Laid open Nos. 50-2130, 56-26271 and 59-28678), a method of managing the residual capacity on the basis of measured capacitance (Japanese Patent Laid-open (Kokai) No. 2-301974), a method of managing the residual capacity on the basis of voltage, current and temperature (Japanese Patent Laid-open (Kokai) No. 2-170372), a method of managing the residual capacity on the basis of internal resistance (Japanese Patent Laid-open (Kokai) No. 3-163375), a method of managing the residual capacity that integrates current and takes charge efficiency, discharge efficiency and temperature characteristics into consideration (Japanese Patent Laid-open (Kokai) No. 63-208773), and methods of determining the residual capacity on the basis of the specific gravity of the electrolytic solution (Japanese Patent Laid-open (Kokai) Nos. 56-24768, 57-88679 and 57-210578). The capacity of a battery is dependent also on the charge and discharge history of the battery. For example, the capacity of nickel-cadmium batteries and nickel-metal hydride batteries decreases due to memory effect if shallow discharge and charge are repeated. In lithium batteries, lithium is accumulated on the positive electrode and in the material forming the negative material to degrade lithium batteries if the balance of charge and discharge is destroyed and, therefore, the power of lithium batteries must be discha rged after due consideration of the charged capacity. It is very important with combination batteries to know the charged capacity. Since the variation of the voltage of nickel-metal hydride batteries, as compared with that of nickel-cadmium batteries, in the final stage of charging is obscure, nickel-metal hydride batteries may be overcharged causing drying of the electrolyte or increase in the internal pressure and entailing safety problems unless nickel-hydrogen batteries are charged after due consideration of the discharge capacity. Since the capacity of batteries is greatly dependent on temperature, charging rate and discharging rate, the charge and discharge history of batteries must be taken into account. Charging systems have been studied with such problems in view. For example, a charging system disclosed in Japanese Patent Laid-open (Kokai) No. 4-308429 charges a battery after detecting the fully discharged condition of the battery from the end voltage or time to solve problems due to memory effect, and a charging system disclosed in Japanese Patent Laid-open (Kokai) No. 61-81139 charges a battery after detecting the fully discharged condition of the battery from the end voltage or time to prevent overcharge. A method of determining the residual capacity of a battery on the basis of the specific gravity of the electrolyte is applicable only to lead-acid batteries. Although a method of managing the residual voltage on the basis of the voltage is effective when applied to lead-acid batteries and lithium batteries the voltage of which varies comparatively widely during charging and discharging, the same is unsuitable for application to nickel-cadmium batteries and nickel-metal hydride batteries the residual capacity of which cannot be determined on the basis of only the voltage. It is difficult to predict the residual capacity accurately on the basis of current and temperature or on the basis of time besides on the basis of voltage under operating condition where discharge rate changes. When managing the residual capacity of a battery on the basis of measured internal resistance or capacitance, it is difficult to determine whether the increase of the internal resistance is due to the deterioration of the battery or whether the increase of the internal resistance is due to the exhaustion of the battery unless the mode of deterioration of the battery is precisely known. The management of the residual capacity on the basis of integrated current, charging efficiency, discharging efficiency and temperature characteristics is able to predict the residual capacity considerably accurately, however, it is difficult to determined the residual capacity unless the capacity of the battery is known when the capacity of the battery is reduced greatly or the self-discharge of the battery is large. Since the prior art methods of determining the residual capacity of batteries are applicable only to specific batteries, respectively and hence each battery requires a specific control. The system proposed to eliminate memory effect (Japanese Patent Laid-open (Kokai)No. 4-308429) and the system proposed to prevent overcharge (Japanese Patent Laid-open (Kokai) No. 61-81139) are intended to suppress the deterioration of a battery and to simplify a charging method when the battery is used as the power supply of portable apparatuses. Such a charge/discharge control method taking the characteristics and the history of batteries has not been proposed. The difference between diurnal power demand and nocturnal power demand has progressively increased in recent years, and diurnal power demand in summer is approaching the upper limit of total power generating ability of power stations. Power storage techniques are effective means for solving problems attributable to the wide daily and seasonal variation of power demand. For example, electric power storage methods published in Denki Gakkai-shi, Vol. 111, No. 3, pp. 185-188 (1992) and such install large-capacity secondary batteries in a substation to store (to charge) surplus electric power generated in the nighttime and to deliver (discharge) the stored power when power demand reaches a peak in the daytime. However, nothing is mentioned about concrete means for the maintenance of soundness of the secondary batteries and the effective use of residual power. JP-A-02 273037 discloses a storage battery monitoring device in which a storage battery is charged by the fluctuating output of a solar battery through a switch while the storage battery supplies electric power to a plurality of loads through switches. Consequently, said document teaches a charging and discharging control of the storage battery in response to the fluctuating output voltage of the solar battery. DE-A-39 39 522 relates to a method for regulating the power supply of a plurality of regenerative energy sources with varying power output feeding into a common electricity supply system. A coupling between each power source and associated battery with the common electricity supply system is established by means of a charge/discharge unit which is driven by a control signal. The document fails to teach to control charging and discharging of the battery on the basis of information comprising measured values for determining the residual electrical power of the battery. WO-A-90/03682 relates to a battery and charging system in which batteries are constructed and arranged to retain a historical record of their use, capacity and discharge/recharge cycles. EP-A-0 225 106 is directed towards a technique for identifying the state of charge of a rechargeable or secondary battery. Said document does not disclose a technique for discharging a battery prior to recharge in order to maintain the soundness of the rechargeable battery. Summary of the Invention Accordingly, it is an object of the present invention to provide a distributed electric electric power storage system capable of maintaining the soundness of secondary batteries and of effectively using residual power. The present invention provides a secondary battery electric power storage system comprising the features of claim 1 and provides a method of operating a secondary battery electric power storage system comprising the features of claim 2. The secondary battery electric power storage system may be provided with at least either a measuring means associated with a plurality of loads to measure information about operating condition or a measuring means associated with the secondary battery to measure information about the surplus electric power. The measuring means may be ordinary measuring means (sensors) suitable for the purpose of measurement. The secondary battery electric power storage system may be characterized also by the connection of a plurality of electric power storage units to the charge/discharge unit, and the connection of the plurality of electric power storage units and the controller by an information transmitting means. The plurality of electric power storage units may be provided with a measuring means for measuring the condition of power storage. The measuring means may be an ordinary measuring means (sensor) suitable for the purpose of measurement. The present invention is applicable to, for example, the secondary batteries of electric vehicles, the second batteries of hotwater supply systems, secondary batteries for illumination, secondary batteries for air-conditioning systems, buildings, factories, apartment houses, cities and general residential buildings provided with power supply facilities, such as emergency power supplies, and transport facilities using electric power. Desirably, the controller is provided with a clock or a timer. The charge/discharge unit capable of being connected to the power system may be connected to the secondary battery, the plurality of loads and the plurality of electric power storage units. The present invention is capable of controlling charge and discharge of the plurality of loads, the plurality of electric power storage units and the secondary battery on the basis of information from the plurality of loads, the plurality of electric power storage units and the secondary battery. Concretely, Necessary power can be measured by a power measuring means on the basis of the operating condition of the plurality of loads. The surplus electric power of the secondary battery, the power storage condition of the plurality of electric power storage units, and the discharge characteristic and the available capacity of the battery are determined, a load to which power is to be fed is selected from among the plurality of loads or the plurality of electric power storage units, and the surplus electric power can be fed through the charge/discharge unit to the selected load. Naturally, power may be stored in the electric power storage units and then power may be fed from the electric power storage units to the plurality of loads. Thus the surplus electric power can be effectively distributed and the residual power can be discharged after determining the amount of available power from the residual power. Therefore, the soundness of the battery can be secured and discharge control can be efficiently carried out. When charging the secondary battery, the residual capacity of the secondary battery is determined before charging, and whether the secondary battery is to be charged or whether the secondary battery is discharged is determined on the basis the residual capacity of the secondary battery. The intrinsic optimum charge and discharge characteristics of the secondary battery, and the power demand of the plurality of loads or the power storage condition of the plurality of electric power storage units are measured and a load to which poser is to be fed is selected. When the plurality of loads has a plurality of loads, it is preferable to measure the power demand of each load. When the plurality of electric power storage units has a plurality of electric power storage units, it is preferable to determine the power storage condition of each electric power storage unit. For example, a load to which power is to be fed and the amount of power to be fed are determined selectively after examining power demand to see if the power demand can be supplied by the secondary battery and to see if the output current density (large-current discharge or small-current discharge) is appropriate to the secondary battery, and then the charge/discharge unit connects the secondary battery to the selected load to feed power to the latter by orders of controller. When the timer is used, the soundness of the battery can be secured and electric power can be efficiently used by accumulating electric power in night period rate hours in the night and using the accumulated electric power in the daytime. More concretely, for example, the secondary battery is charged through the charge/discharge unit with electric power during night period rate hours, the surplus electric power of the secondary battery is fed (discharged) to the charge/discharge unit connected to the secondary battery, and electric power is fed (discharged) through the charge/discharge unit to the plurality of electric power storage units, the plurality of loads connected to the plurality of electric power storage units and the plurality of loads connected to the charge/discharge unit in day period rate hours. Inexpensive night period rate electric power can be stored and used, and then the stored electric power can be sold through the charge/discharge unit that can be connected to the secondary battery to consumers in day. When selling the stored electric power, it is preferable to determine the power demand of the plurality of loads on the basis of information about operating condition received from a plurality of electric power storage units or the plurality of loads by the controller connected to the charge/discharge unit connected to the secondary batteries, to select a load to which electric power is to be fed, taking into consideration the available power capacity of the secondary battery, and to feed the surplus electric power of the secondary battery through the charge/discharge unit to the selected load. The soundness and the original power capacity of the battery can be known by discharging the surplus electric power. The original power capacity of the battery can be known from charged capacity after discharge of nearly 100% of the full capacity of the battery (nearly exhausted state). It is desirable, if possible, to know the capacity of the battery every time the battery is discharged or charged to know the soundness of the battery. Desirably, the secondary battery employed in the present invention is provided with, for example, measuring means (sensors) for acquiring data, an A/D converter for converting the data acquired by the measuring means and giving the converted data to a computer, a memory storing data on the charge and discharge history, arithmetic programs, an indication program and data on the standard characteristics, and a controller for processing the information stored in the memory and information placed into the memory by external devices. These components may be integrally combined with the battery. For example, the arithmetic programs and the indication program are an arithmetic program defining a procedure for determining discharged capacity by integrating discharge current, an arithmetic program defining a procedure for determining charged capacity by integrating charge current, an arithmetic program defining a procedure for determining converted charged capacity, i.e., available capacity under a discharge rate and a temperature condition at the time when discharge current data is received from the discharge efficiency and the temperature characteristic, determined by converting charged capacity in a real-time mode, an arithmetic program defining a procedure for determining surplus electric power discharged capacity by integrating surplus electric power discharge current data, and an indication program defining a procedure for indicating the surplus electric power discharged capacity determined by the procedure defined by the arithmetic program. Discharge current data, discharge voltage data and discharge temperature data on the discharge history, are measured by measuring means (sensors) when the secondary battery feeds electric power to the loads, and the measured data are transferred through an A/D converter to a computer provided with a memory and a controller. Charging current data, charge voltage data and charge temperature data on the charge history, are measured when the secondary battery is charged with electric power through the charge/discharge unit connected to the secondary battery in the night, and the measured data are transferred through the A/D converter to the computer. Surplus electric power discharge current data, surplus electric power discharge voltage data and surplus electric power discharge temperature on surplus electric power discharge history, are measured by sensors when the secondary battery discharges surplus electric power through the charge/discharge unit connected to the secondary battery, and the measured data are transferred through the A/D converter to the computer. The soundness of the battery can be known from those measured data. A discharged capacity discharged to the loads and a surplus electric power discharged capacity discharged to the charge/discharge unit can be known from the discharge current data and the surplus electric power discharge current, and the discharge operation of the secondary battery can be controlled so that the battery may not be discharged to a voltage below the final discharge voltage, i.e., so that the secondary battery may not be overdischarged, by using the discharge voltage data and the surplus electric power discharge voltage data. The charged capacity charged into the secondary battery can be known from the charge current data. The increase of the voltage of a lithium battery to its final charge voltage can be known from the charge voltage data and hence the overcharge of the lithium battery can be prevented. The final charge voltage of a nickel-cadmium battery can be known from the variation of the voltage in the final stage of charging operation and hence charging can be terminated before the nickel -cadmium battery is overcharged. Unlike a nickel-cadmium battery, a nickel-metal hydride battery has a small voltage variation in the final stage of charging. Therefore, it is desirable to take temperature variation into consideration in addition to voltage variation in determining the time to stop charging to prevent overcharge. As regards other kinds of batteries, abnormal rise of temperature is an indication of the abnormal condition of the batteries. Since charging efficiency and discharging efficiency are dependent on temperature, it is preferable to use temperature data in correcting charged capacity and discharged capacity because charged capacity and discharged capacity are dependent on temperature. Desirably, the aforesaid data are corrected for charging rate, discharging rate and temperature. The standard characteristic data, such as the inherent charging efficiency, discharging efficiency and temperature characteristic, of the secondary battery are stored in the memory. The charge current data and the discharge current data given to the A/D converter are integrated to determine charged capacity and discharged capacity. The charged capacity is converted in a real-time mode into a converted charged capacity, i.e. , a capacity available at the discharge rate and under the temperature condition at the time when the discharge current data is obtained, on the basis of the charging efficiency, the discharging efficiency and the temperature characteristic stored in the memory. The discharged capacity is subtracted from the converted charged capacity determined in the real-time mode to determine a residual capacity. Thus, the residual capacity at the current temperature and at the current discharge rate can be indicated. For example, since the possible distance of travel of an electric vehicle is dependent on the condition of the road, namely, the possible distance of travel along a downhill is longer than that along an uphill, the possible distance of travel can be estimated on the basis of the discharging efficiency according to the condition of the road, and the estimated possible distance of travel can be indicated. Since the discharging efficiency is dependent on temperature and is subject to seasonal variation, a possible distance of travel determined by taking into consideration the temperature characteristic can be indicated. Surplus electric power discharge capacity can be determined by integrating the surplus electric power discharge current data and the surplus electric power discharge capacity can be indicated. Since the surplus electric power discharge capacity is selling electric energy for sale, the surplus electric power discharge capacity is transferred to the charge/discharge unit connected to the secondary battery and the power system, the plurality of electric power storage units or the plurality of loads. Preferably the charge/discharge unit is capable of measuring and indicating the capacity. It is desirable to discharge the surplus electric power from the secondary battery in a mode conforming to optimum discharge conditions for the secondary battery to secure the soundness of the secondary battery and to extend the cycle life of the secondary battery. It is preferable to discharge the surplus electric power from the secondary battery in conformance with optimum discharge conditions stored in the memory when the surplus electric power is discharged to the charge/discharge unit connected to the secondary battery and connectable to the power system. The optimum charge and discharge conditions are pieces of information about a discharge method specifying, for example, at least one of maximum discharge capacity, discharge current, discharge voltage, discharge time and a lower limit voltage. More concretely, discharging conditions as battery characteristic data including discharge modes, such as a constant-current discharge mode, a constant-voltage discharge mode, a pulse discharge mode and combinations of those discharge modes, an optimum current, an optimum voltage, and discharge cutting mode, such as a time-cut mote and a voltage-cut mode, is stored beforehand in the memory. Charging information specifying charging conditions similar to the discharging conditions are used for charging. If even one of the batteries of a battery set is deteriorated, the deteriorated battery will be overdischarged and the cycle life of the same is shorted significantly. It is desirable to use a control method determined by taking into consideration the type the scale and the security of the battery to execute time-cut or voltage-cut on the basis of monitored voltage variation. The available discharge capacity that can be discharged in the future (next) discharge cycle and the cycle life of the secondary battery can be estimated from the past charge capacities of the secondary battery and the mode of change of charged capacity. Causes of reduction of the capacity of the secondary battery, such as the exhaustion of the electrolyte, the deterioration of the positive electrode and the deterioration of the negative electrode, can be determined from the mode of reduction of the capacity. Overdischarge of the battery can be prevented by determining the available discharge capacity and the possible discharge time of the battery and discharging the surplus power accordingly. Similarly, the secondary battery is charged in a mode conforming to the optimum charge conditions stored in the memory when charging the secondary battery through the charge/discharge unit at night, in which overcharge of the secondary battery can be prevented by determining the available discharge capacity and the possible discharge time, and charging the secondary battery accordingly. It is desirable to determine beforehand surplus electric power discharge hours. For example, the discharge of the surplus electric power is interrupted when the surplus electric power is not discharged completely. The charge capacity determined in the preceding charge cycle is used as charge capacity. Preferably, the surplus electric power discharge capacity is determined on the basis of the residual capacity, taking into consideration the type of the battery to secure the soundness of the battery. For example, since a nickel-cadmium battery and a nickel-metal hydride battery is subject to large self-discharge and has a memory effect, it is preferable to discharge 95% to 100% of a discharge capacity in a mode conforming to optimum discharge conditions revised for the future available discharge capacity of the battery when the residual capacity is in the range of 0% to 85% of the charge capacity. When the residual capacity is 85% or above of the charge capacity, it is better not to discharge the surplus electric power. Thus, charging and discharging are simplified and the memory effect can be prevented. Since a lithium battery or the like is subject to a comparatively small self-discharge and the cycle cycle life of a lithium battery or the like is reduced when the same is discharged deep, it is desirable to discharge the surplus electric power corresponding to 80% to 95% of the discharge capacity in a mode conforming to optimum discharge conditions revised for the available discharge capacity of the battery, when the residual capacity is in the range of 5% to 80% of the charge capacity. When the residual capacity is less than t% of the charge capacity, it is better not to discharge the surplus electric power because further discharge reduces the cycle cycle life of the battery. When the residual capacity is 80% or above of the charge capacity, it is desirable not to discharge the surplus electric power to simplify charging and discharging. The whole capacity of battery can be known at every discharge and the residual capacity of the battery can be accurately determined when the surplus electric power is discharged with the performance of the battery being taking into consideration. The cycle life of the battery and the future capacity of the battery can be estimated from the whole capacity of the battery and the residual capacity. Accordingly, charge/discharge control can be carried out according to the change of the performance of the battery and thereby the soundless of the battery is enhanced. Charging of the battery with the inexpensive night period rate electric power in the nighttime and using the surplus electric power in the daytime are economically advantageous. Brief Description of the Drawings Fig. 1 is a block diagram of a secondary battery electoric power storage system in accordance with the present invention; Fig. 2 is a block diagram of a secondary battery electoric power storage system in accordance with the present invention; Fig. 3 is a block diagram of a secondary battery electoric power storage system in accordance with the present invention; Fig. 4 is a block diagram of a secondary battery electoric power storage system in accordance with the present invention; Fig. 5 is a flow chart of a procedure for controlling a secondary battery, in accordance with the present invention; Fig. 6 is a graph showing the charge/discharge cycle characteristics of secondary batteries in a second embodiment according to the present invention; Fig. 7 is a graph comparatively showing residual capacity indications and actual residual capacities in the second embodiment according to the present invention; Fig. 8 is a graph showing the charge/discharge cycles of secondary batteries in a third embodiment according to the present invention; Fig. 9 is a graph comparatively showing residual capacity indications and residual capacities in a comparative example 1; and Fig. 10 is a graph showing the charge/discharge cycle characteristics of secondary batteries in the comparative example 1. Detailed Description of the Preferred Embodiments Preferred embodiments of the present invention will be described in detail hereinafter. The embodiments are illustrative and not restrictive. First Embodiment Fig. 1 shows a secondary battery electric power storage system in a first embodiment according to the present invention. Although the first embodiment is one of the best modes for carrying out the invention, the present invention is not limited thereto. Referring to Fig. 1, a load 1 and a charge/discharge unit 2 connectable to a power system are connected to a secondary battery 3. A plurality of electric power storage units 4 are placed in parallel combination with the charge/discharge unit 2 connected to a power supply system. A plurality of loads 5 are connected to the charge/discharge unit 2 or the electric power storage units 4. A signal line 30 indicated by broken lines is connected to a controller 7 included in the charge/discharge unit 2. Ditecting device 40 detect a residual electric power in the secondry battery. When necessary, the secondary battery electric power storage system is provided with a means, not shown in Figs. 1 to 4, for supplying electric power directly from the power supply system to the loads 5 or the electric power storage units 4, bypassing the charge/discharge unit 2. When the secondary battery electric power storage system is provided with such a means, it is desirable that the secondary battery electric power storage system is provided with a signal line for transmitting signals representing measured values, such as the amount of electric power used to the controller 7 of the charge/discharge unit 2. Amounts of electric power applied to and delivered from the secondary battery 3 and the electric power storage units 4 are measured by measuring devices and measured data is sent through the signal line to the controller 7 of the charge/discharge unit 2. The secondary battery 3, the electric power storage units 4 and the loads 5 are provided with sensors (measuring means). First the secondary battery 3 is connected to the charge/discharge unit 2 to charge the secondary battery 3 with night period rate electric power. After the secondary battery has been charged, the secondary battery 3 is connected to the load 1 to discharge the secondary battery 3. After discharging, the secondary battery 3 is disconnected from the load 1 and connected to the charge/discharge unit 2 to discharge the surplus electric power of the secondary batter 3 in the day period rate hours. A controller 6 included in the secondary battery 3 controls the secondary batter 3 for discharging the surplus electric power remaining after charging the load 1, i.e., the residual capacity in a mode conforming to optimum discharging conditions in the day period rate hours. The discharge of the surplus electric power is stopped in the night period rate hours and the secondary battery is charged. In this state, the charge capacity does not correspond to the capacity of the battery because the battery has the residual capacity. Therefore, the charge capacity used in the preceding charge cycle is used as the charge capacity to be taking into consideration in indicating the residual capacity. The controller 7 of the charge/discharge unit 2 measures the amounts of electric power required by the electric power storage units 4 or the loads 5, selects the electric power storage unit or the load and controls the secondary battery 3 to supply the surplus electric power to the selected electric power storage unit or the load. Fig. 2 shows an embodiment of the present invention. In this embodiment, a load 1 and a charge/discharge unit 2 connectable to a power system are connected to a secondary battery 3, and electric power storage units 4 are placed in parallel combination with the charge/discharge unit 2. Fig. 3 shows an embodiment of the present invention. In this embodiment, a load 1 and a charge/discharge unit 2 connectable to a power system are connected to a secondary battery 3, and loads 5 are connected to the charge/discharge unit 2. Fig. 4 shows an embodiment of the present invention. In this embodiment, a load 1 and a charge/discharge unit 2 connectable to a power system are connected to a secondary battery 3, and series circuits each of a electric power storage unit 4 and a load 5 are placed in parallel connection with the charge/discharge unit 2. Second Embodiment Fig. 5 is a flow chart of a control procedure to be carried out by the controllers 6 of the secondary batteries 3 of Figs. 1 to 4. The control procedure will be described hereinafter with reference to Figs. 1 to 5. The secondary battery 3 is provided with the controller 6, an A/d converter 8 and a memory 9. The memory 9 is capable of storing standard characteristic data on the intrinsic characteristics of the secondary battery 3 including charging efficiencys 10, discharging efficiencys 11, temperature characteristics 12 and optimum charge and discharge conditions 13. The optimum charge and discharge conditions 13 specify charge and discharge modes, such as a constant-current charge mode, constant discharge mode, a constant-voltage discharge mode and a constant -voltage discharge mode, currents and voltages, cut voltages, charge capacities, discharge capacities, charge times and discharge times. When the secondary battery 3 is connected to the charge/discharge unit 2 for charging, the secondary battery 3 is controlled for charging according to the optimum charge conditions. At every charge cycle, an A/D converter receives charge operation data including charge current data 14, charge voltage data 15 and charge temperature data 16. The charge voltage data 15 is necessary for terminating charging at a cut voltage. The charge temperature data 16 is used for detecting the final charge stage for a nickel-metal hydride battery. The temperature data 15 is used also for detecting the abnormal condition of the battery when the temperature of the battery rises abnormally. The charge current data 14 is integrated to obtain a charged capacity. When the secondary battery 3 is connected to the load 1 and electric power stored in the secondary battery 3 is discharged into the load 1, the A/D converter 8 receives discharge operation data including discharge current data 17, discharge voltage data 18 and discharge temperature data 19. The discharge voltage data 18 is necessary for terminating discharge at a cut voltage. The discharge current data 17 is integrated to obtain a discharged capacity. The discharged capacity is converted in a real-time mode for a discharging rate and a temperature represented by the discharge temperature data 19 into an available discharge capacity, i.e., converted charge capacity, for determining a residual capacity. The residual capacity is obtained by subtracting the discharged capacity from the converted charge capacity. When the secondary battery 3 is connected to the charge/discharge unit 2 to discharge the surplus electric power, the discharge operation of the secondary battery 3 is controlled according to the optimum charge and discharge conditions 13. At every surplus electric power discharging operation, the A/D converter 8 receives surplus electric power discharge operation data including surplus electric power discharge current data 20, surplus electric power discharge voltage data 21 and surplus electric power temperature data 22. The surplus electric power discharge current data 20 is integrated to obtain a surplus electric power discharge capacity, and the surplus electric power discharge capacity is indicated. The surplus electric power discharge capacity can be transferred through the charge/discharge unit 2 to the electric power storage units 4 and the loads 5. When selling the surplus electric power, it is preferable to indicate an amount of money corresponding to the surplus electric power when necessary. Fig. 6 shows the charge and discharge cycle characteristics of secondary batteries and Fig. 7 shows the variation of the difference between residual capacity indication and actual residual capacity with the number of charge and discharge cycles. The capacities of a lead-acid battery B, a nickel-cadmium battery D, a nickel-hydrogen battery A and lithium battery E decrease slightly as the number of charge and discharge cycles increases, and the possible numbers of charge and discharge cycles for those batteries are not less than 1000. The difference between the residual capacity indication and the actual residual capacity is very small. Third Embodiment When a secondary battery 3 is a nickel-cadmium battery or a nickel-metal hydride battery , a controller 6 controls the secondary battery 3 for discharging so that 95% to 100% of a discharge capacity corresponding to an available discharge capacity is discharged in a mode conforming to optimum discharge conditions when the residual capacity is 0 to 85% of the charge capacity of the secondary battery 3. If the residual capacity is 85% or above of the charge capacity, surplus electric power discharge is not performed. When the secondary battery 3 is a lithium battery, the controller 6 controls the secondary battery 3 so that 80% to 95% of a discharge capacity corresponding to an available discharge capacity is discharged in a mode conforming to optimum discharge conditions when the residual capacity is 5% to 80% of the charge capacity. When the residual capacity is 5% or below of the charge capacity, surplus electric power discharge is not performed. When the residual capacity is 80% or above of the charge capacity, surplus electric power discharge is not performed. Fig. 8 shows the charge and discharge cycle characteristics of batteries under the control operation of the controller. The capacities of a nickel-cadmium battery, a nickel-metal hydride battery and a lithium battery decrease scarcely as the number of charge and discharge cycles increases. The possible numbers of charge and discharge cycles for those batteries are not less than 1200. Comparative Example 1 In a secondary battery electric power storage system in a comparative example 1, a load 1 and a charge/discharge unit 2 are connected to a secondary battery 3. The secondary battery 3 is connected to the charge/discharge unit 2 and the secondary battery 3 is charged with night period rate electric power. Then, the secondary battery 3 is connected to the load 1 and the electric power stored in the secondary battery 3 is discharged into the load 1. After discharging, the secondary battery 3 is disconnected from the load 1 and is connected to the charge/discharge unit 2 to charge the secondary battery 3 with night period rate electric power. The residual capacity of the secondary battery is determined by subtracting a discharged capacity from an initial capacity. Fig. 9 shows the difference between residual capacity indication and actual residual capacity. As is obvious from Fig. 9, the difference increases as the number of charge and discharge cycles increases and, consequently, accurate residual capacity indication is impossible. Fig. 10 shows the charge and discharge cycle characteristics of batteries. As is obvious from Fig. 10, the capacities of a lead-acid battery, a nickel-cadmium battery, a nickel-metal hydride battery and a lithium battery decrease greatly as the number of charge and discharge cycles increases, and the lives of those batteries are in the range of 500 to 700 charge and discharge cycles. As is apparent from the foregoing description, according to the present invention, the soundness of the battery can be secured, and charging and discharging are carried out efficiently. The secondary battery can be charged with inexpensive night period rate electric power in the night and the surplus electric power can be supplied to loads in the day time.

Claims: A secondary battery electric power storage system comprising: a charge/discharge unit (2) connectable to a power system and at least one load (5) and/or at least one electric power storage unit (4); and means for connecting said charge/discharge unit to a secondary battery (3), said secondary battery (3) being connectable to a secondary battery load (1), characterized in that said system further comprises means (7) for determining an available discharge capacity of said secondary battery and control means (6) for controlling said system on the basis of information received from the at least one load (5) and/or at least one electric power storage unit (4), such that electric power in the secondary battery (3) is discharged to a selected at least one of said at least one load and/or at least one electric power storage unit when a determined surplus residual electric power of said secondary battery is within a predetermined range, 0 % to 85 % for a NiCd or NiH battery type and 5 % to 80 % for a Li battery type, prior to the charging of said secondary battery (3). A method of operating a secondary battery electric power storage system comprising: a charge/discharge unit (2) connectable to a power system and at least one load (5) and/or at least one electric power storage unit (4); and means for connecting said charge/discharge unit to a secondary battery(3), said secondary battery (3) being connectable to a secondary battery load (1), characterized in that the method comprises: receiving information from the at least one load (5) and/or the at least one electric power storage unit (4); obtaining a measure of a surplus residual electric power of said secondry battery; and discharging electric power in the secondary battery (3) to a selected at least one of said at least one load (5) and/or at least one electric power storage unit (4) when the residual electric power is within a predetermined range, 0 % to 85 % for a NiCd or NiH battery type and 5 % to 80 % for a Li battery type, prior to charging said secondary battery (3). An electric vehicle using a secondary battery electric power storage as defined in claim 1. An electric vehicle comprising a secondary battery (3), and secondary battery load (1) being connectable to said secondary battery (3), characterized in that said vehicle further comprises means (7) for determining an available discharge capacity of said secondary battery such that electric power in the secondary battery (3) is discharged when a determined surplus residual electric power of said secondary battery is within a predetermined surplus residual electric power of said secondary battery is within a predetermined range, 0 % to 85 % for a NiCd or NiH battery type and 5 % to 80 % for a Li battery type prior to the charging of said secondary battery (3).

Inventors: HORIBA TATSUO, IKAWA KYOKO, HORIBA, TATSUO, IKAWA, KYOKO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1994
Main classification: H02J7
IPC codes: H02J7, H02J3, H01M10, H02J13